Embodiments of the invention relate generally to power supplies and, more particularly, to an apparatus for supplying multi-channel, isolated, and stable DC power. While embodiments of the invention may be described with respect to a magnetic resonant imaging system, one skilled in the art will recognize that the invention may be used in any device where multiple channels of isolated, stable DC power is desired.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Typically, the magnetic field gradients Gx, Gy and Gz are generated by three separate gradient coils, respectively. Each coil is driven by a gradient amplifier, whose power output determines the peak amplitude and slew rate of the magnetic gradient. The peak amplitude primarily affects image resolution, while the slew rate most affects image acquisition time.
Gradient amplifiers comprise a major portion of the total cost of an MR imaging system. Typically, MR imaging systems have three gradient amplifiers, one for each axis in the Cartesian coordinate system (i.e. x, y, and z). Each gradient amplifier typically uses several galvanically isolated DC power supplies. For example, a typical MR imaging system may have 12 isolated DC channels supplying power to the gradient amplifiers. As the size of gradient amplifiers increase to provide improved imaging functionality, the size and cost of DC power supplies for those amplifiers also increase.
In a typical MR imaging system, the multiple DC channels needed to provide power for the operation of gradient amplifiers are typically supplied by DC-to-DC resonant converters or some combination of rectifiers and buck converters. For DC-to-DC resonant converters, a separate resonant converter is used for each DC channel. Isolation of the output from the input typically calls for a transformer to be added to each resonant converter. While it is possible for some resonant converter circuits to share a transformer, the 12 DC channels in a typical MR imaging system may result in multiple transformers in the system power supply. Also, for high-voltage operation, resonant converters use specialized components increasing the cost of the system. Resonant converters, which operate at high frequencies, include switches and rectifier diodes capable of handling elevated power levels at high frequencies, making the components more difficult to manufacture, and therefore more expensive. Similarly, the high-frequency transformers needed to operate resonant converters are also expensive and difficult to manufacture.
Buck converters, which typically use PWM-switching to modulate DC voltage input levels to the desired DC output level, also have some disadvantages with regard to size and cost. Like resonant converters, buck converters require a transformer to isolate the output from the input. A typical MR imaging system with 12 buck converters may have a number of transformers for power supply isolation. Additionally, each buck regulator may require an unregulated rectifier to convert the AC input into a DC input for the buck converter, an IGBT/Diode, and an inductor for proper operation of each buck regulator. For a typical MR imaging system using buck converters or resonant converters to supply DC power to its gradient amplifiers, the size, complexity, and cost of those power supplies increase significantly with the number of DC channels provided.
One of the factors that determine the size and cost of power supply components is their power rating, which indicates the maximum power that can safely flow through the device. To the extent that a power supply can be designed to minimize the required power ratings of its components, the material costs required to construct such a power supply are reduced. Another factor influencing component cost is the range of frequency operation. In general, components designed to operate at high frequencies are more costly than components designed to operate at lower frequencies.
It would therefore be desirable to have an apparatus to supply multiple channels of stable, isolated DC power in a cost effective manner.